1474000 - Undefined lazy properties are exposed and then disappear

Status: RESOLVED WONTFIX

(WebExtensions :: General, defect)

Description

[Oriol Brufau [:Oriol]]

Attachment: test.xpi

Install attached add-on in about:debugging.

It runs this code:

    let obj = (window.browser || window.chrome).browserAction;
    let prop = "ColorArray";
    let ownKeys = Reflect.ownKeys(obj);
    console.log(prop + " in ownKeys: " + ownKeys.includes(prop));
    let descr = Reflect.getOwnPropertyDescriptor(obj, prop);
    console.log(prop + " descriptor: " + JSON.stringify(descr));
    let value = obj[prop];
    console.log(prop + " value: " + value);
    ownKeys = Reflect.ownKeys(obj);
    console.log(prop + " in ownKeys: " + ownKeys.includes(prop));
    descr = Reflect.getOwnPropertyDescriptor(obj, prop);
    console.log(prop + " descriptor: " + JSON.stringify(descr));

The console shows

    ColorArray in ownKeys: true
    ColorArray descriptor: {"enumerable":true,"configurable":true}
    ColorArray value: undefined
    ColorArray in ownKeys: false
    ColorArray descriptor: undefined

So there is a lazy property which is deleted after being read.
It shouldn't be there in the first place, just like in Chrome:

    ColorArray in ownKeys: false
    ColorArray descriptor: undefined
    ColorArray value: undefined
    ColorArray in ownKeys: false
    ColorArray descriptor: undefined

Regression window: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=855e6b2f6199189f37cea093cbdd1735e297e8aa&tochange=09ef6a4f41186ea6951830d1506f741005dcc46f

I guess bug 1338409.

Comment 1

[Kris Maglione [:kmag]]

Yes, this is expected.

Comment 2

[Oriol Brufau [:Oriol]]

I know lazy properties are expected, but I don't see the need for lazy properties which will then be removed.
Can't these be detected beforehand?

It's annoying when I'm debugging an extension and autocomplete offers useless properties.

Comment 3

[Kris Maglione [:kmag]]

It could be detected beforehand, but that would be expensive. The whole point of lazy properties is to avoid that expense.

Comment 4

[Oriol Brufau [:Oriol]]

Sure, I meant some way to check which avoids expensive work if the result is not going to be undefined.
For example, the problematic properties seem the "types" from the schema.
Can't these be filtered out directly? Or is there any type which does have a value?